Steel cords are widely known to reinforce rubber products such as tires and conveyor belts. To a lesser extent, steel cords are also known to reinforce thermoplastic elastomers such as polyurethanes. The adhesion mechanism between the steel cords and the rubber products is substantially different from the adhesion mechanism between steel cords and thermoplastic elastomers. The adhesion between steel cords and rubber is mainly a chemical adhesion based on the bonds created during vulcanization between a conventional copper alloy coating on the steel cord and the rubber. Although chemical adhesion is not excluded between steel cords and thermoplastic elastomers, the adhesion mechanism is substantially based on a mechanical anchoring between the steel cords and the matrix.
Due to this basic difference, steel cords for rubber reinforcement have known another evolution than steel cords for reinforcement of thermoplastic elastomers.
With respect to rubber reinforcement, there has been an evolution towards single-strand cords such as compact cords, lesser filaments in one steel cord (even ending up in single filament reinforcements), thicker filament diameters.
With respect to reinforcement of thermoplastic elastomers, the multi-strand steel cords, i.e. steel cords comprising more than one strand, have remained the standard because the rougher outer surface of such a multi-strand steel cord offers more mechanical anchoring in the matrix than a single-strand steel cord.
Another consequence of the different adhesion mechanism between steel cords adapted for the reinforcement of rubber products and steel cords adapted for the reinforcement of thermoplastic elastomers, is the type of coating applied to the steel filaments. Whereas steel cords for rubber reinforcement, particularly for tires, have a conventional copper alloy coating such as brass, steel cords for the reinforcement of thermoplastic elastomers conveniently have a zinc or zinc alloy coating. Such a zinc coating, however, has its drawbacks.
A first drawback is that a suitable level of corrosion resistance is difficult to combine with an acceptable level of fatigue resistance. Indeed increasing the thickness of the zinc coating leads to an increase of the corrosion resistance, which is an advantage, and to a decrease in fatigue resistance, which is a disadvantage, and vice versa.
A second drawback is that a zinc coating creates a lot of zinc dust and zinc particles during the downstream working of the zinc coated filaments such as the cold drawing and the twisting into the strands and cord.